High resolution radiation sensor based on single polysilicon floating gate array

ABSTRACT

A method for radiation dosage measurement includes: (1) exposing a plurality of single-poly floating gate sensor cells to radiation; (2) measuring threshold voltage differences between logical pairs of the exposed sensor cells using differential read operations, wherein the sensor cells of each logical pair are separated by a distance large enough that radiation impinging on one of the sensor cells does not influence the other sensor cell; (3) determining whether each logical pair of exposed sensor cells is influenced by exposure to the radiation in response to the corresponding measured threshold voltage difference; and (4) determining a dosage of the radiation in response to the number of logical pairs of the exposed sensor cells determined to be influenced by exposure to the radiation. A non-radiation influenced threshold voltage shift may be measured and used in determining whether each logical pair of exposed sensor cells is influenced by radiation exposure.

FIELD OF THE INVENTION

The present invention relates to a high-resolution radiation sensorarray that implements low-capacitance floating gate cells having asingle polysilicon layer. The present invention further relates tomethods for operating the radiation sensor array to enable sensing ofboth low dose and high dose radiation exposure, wherein the methodsprovide compensation for natural retention loss and temperature effects.

RELATED ART

Passive solid state dosimeters often use floating gates. The floatinggates are electrically isolated and charged before taking measurements.A dosimeter may have a gas filled chamber located over a floating gatedevice as described in U.S. Pat. No. 5,739,541 to Kahilainen, or over anon-volatile memory (NVM) cell as described in U.S. Pat. No. 6,172,368to Tarr et al. (hereinafter, “the Tarr '368 Patent”). To define thethreshold voltage (Vt) change in a floating gate based sensor due toradiation absorption, comparison of the sensor drain current with areference device current is needed. In the Tarr '368 Patent, thiscomparison was performed using a pair of matched double polysiliconfloating gate transistors, wherein: (1) different voltages are appliedto the control gates of these transistors, or (2) the floating gates ofthese transistors are charged to opposite polarities. The pair offloating gate transistors is then exposed to the same radiation, and adifference in drain currents through these transistors is then measured.However, charge loss from the floating gates of these transistors canoccur for other reasons than ionizing radiation, specifically, bythermal excitation (which is typical for all floating gate devices) orby the presence of ions in surrounding oxides. The construction of thefloating gate transistors and the above-described method of operation donot allow for compensation of this charge loss. Moreover, floating gatetransistors having their floating gates charged to significantlydifferent potentials will exhibit different natural retention losses. Asa result, the type of floating gate dosimeter described by the Tarr '368patent has significant limitations.

U.S. Pat. No. 8,519,345 to Arsalan et al. describes a sensor thatincludes a single polysilicon floating gate sensor device and a singlepolysilicon floating gate reference device, which have very differentlayouts, geometries and sizes. The single polysilicon floating gatereference device is not sensitive to radiation (due to the lack of anextension of the floating gate over an adjacent field dielectric). Thereference device serves to compensate for environmental influences(e.g., temperature) on the sensor DC current. As for the retentionperformance, significantly different layouts in the sensor device andthe reference device lead to different retention performance. Thus, inlong term measurements (especially for low radiation dose rates),matching of the reference device and the sensor device becomesincorrect.

In U.S. Pat. No. 8,791,418 to Visconti et al., a two dimensional arrayof memory cells is used to implement a spatial dosimeter. The change inthe threshold voltage of each of the cells, as a result of large doseradiation exposure, may be used to calculate the dose seen at each cell,allowing dose profiles in two dimensions with sub-micrometer resolution.However, this radiation sensor will only work with exposure to largeradiation doses.

U.S. Pat. No. 9,429,661 to Valentino et al. describes techniques toencapsulate individual ionizing radiation sensor elements mounted on aprinted circuit board with a radiation attenuating material thatprovides a ‘filtration bubble’ around the sensor element.

None of the above-mentioned references address the issues associatedwith measuring very low radiation doses with floating gate sensorarrays. When radiation doses at a level of 1 milli-rad (mrad) and beloware considered, the number of Gamma photons or Alpha particles impingingon the sensor area is much smaller than the number of cells/pixels ofthe sensor. For example, for a 1 mrad dose, only about two 1 MeV photonsare present for a sensor area of 1 mm². The structures and methodsdescribed in the above-mentioned references are incapable of measuringsuch low radiation doses.

It would therefore be desirable to have an improved radiation sensorthat is capable of measuring both very low radiation doses and highradiation doses. It would further be desirable for such an improvedradiation sensor to be fabricated in accordance with a conventional CMOSprocess that includes a single polysilicon layer. It would further bedesirable for such an improved radiation sensor to be able todistinguish between radiation having different energies. It wouldfurther be desirable for such an improved radiation sensor to be able tocompensate for non-radiation based leakage factors such as temperatureand natural retention loss of charge from the floating gate. It wouldfurther be desirable to have low-capacitance floating gate transistorstructures for use in the improved radiation sensor.

SUMMARY

Accordingly, the present invention provides a method for measuringradiation dosage that includes exposing a plurality of single-polyfloating gate sensor cells to radiation. The sensor cells are logicallygrouped into a plurality of pairs, wherein the two sensor cells of eachlogical pair are separated by a physical distance, such that a singleenergetic particle/photon that impinges on one of the sensor cells ofthe logical pair does not influence the other sensor cell of the logicalpair.

After radiation exposure, a plurality of differential read operationsare performed, wherein each differential read operation accesses acorresponding logical pair of the exposed sensor cells, therebyidentifying a threshold voltage difference between the logical pair ofthe exposed sensor cells. The identified threshold voltage differencesare used to determine whether each logical pair of exposed sensor cellshas been influenced by exposure to the radiation. For example, if asingle energetic particle/photon impinges on only one sensor cell of alogical pair, and such an impingement would result in a known thresholdvoltage difference between the sensor cells of the logical pair, then alogical pair of exposed sensor cells that has an identified thresholdvoltage difference that corresponds with this known threshold voltagedifference may be identified as an influenced logical pair of sensorcells. For low dosage radiation exposure (e.g., 50% or fewer of thesensor cells receives an impinging energetic particle/photon) the dosageof the radiation is determined based on the number of identifiedinfluenced logical pairs of sensor cells.

In accordance with one embodiment, the initial threshold voltage (afterinitialization and prior to radiation exposure) of each of the pluralityof sensor cells is determined. For example, the threshold voltage ofeach of the sensor cells may be determined by performing a differentialread operation with a radiation-insensitive reference cell. The initialthreshold voltage of each sensor cell is used to calculate an averageinitial threshold voltage of all of the sensor cells. The post-exposurethreshold voltage of each of the sensor cells is also determined (e.g.,by performing a differential read operation with a radiation-insensitivereference cell). The post-exposure threshold voltage of each sensor cellis used to calculate an average post-exposure threshold voltage of allof the sensor cells. An average threshold voltage shift is determined bycalculating the difference between the average initial threshold voltageand the average post-exposure threshold voltage.

In one embodiment, the average threshold voltage shift is used todetermine whether each logical pair of the exposed sensor cells isinfluenced by exposure to radiation. For example, a logical pair ofexposed sensor cells is only determined to be influenced by exposure toradiation if these exposed sensor cells: (1) exhibit the determinedaverage threshold voltage shift, and (2) exhibit the above-describedknown threshold voltage difference.

In accordance with another embodiment, the each of the plurality ofsensor cells includes a read transistor that is coupled to thecorresponding floating gate. In this embodiment, each differential readoperation includes coupling the read transistors of the logical pair ofsensor cells to a constant current source (that comprises an integralpart of a common sense amplifier) and biasing the control gates of thesensor cells such that the same current flows through both sensor cells,wherein the voltage difference between control gates of the sensor cellsis read out as a threshold voltage difference.

In accordance with another embodiment, the charges stored by thefloating gates of the sensor cells are initialized (by a program orerase operation) prior to exposing sensor cells to radiation, such thatthe dielectric interface traps of these sensor cells are all filled (orall empty) before each read operation. This advantageously ensures thesuppression of 1/f noise in the read transistors of the sensor cells forlow dose radiation measurements (because this initial state does notsignificantly change in response to low dose radiation exposure, so thattraps remain in the empty/filled state after irradiation because thethreshold voltage shift (i.e., the change in the charge stored by thefloating gate) is small.

In accordance with another embodiment, the number of logical pairs ofexposed sensor cells determined to be influenced by exposure to theradiation is compared with a threshold number (e.g., half of the totalnumber of sensor cells). If the number of influenced logical pairs ofsensor cells is less than the threshold number, then the exposure isidentified as a low dose exposure, and the dose is calculated based onthe number of influenced logical pairs of sensor cells. However, if thenumber of influenced logical pairs is greater than or equal to thethreshold number, then the exposure is identified as a high doseexposure, and the dose is calculated based on the difference between theaverage initial threshold voltage of the sensor cells and the averagepost-exposure threshold voltage of the sensor cells.

In accordance with yet another embodiment, the plurality of sensor cellsare arranged in an array that includes a first sub-array and a secondsub-array, where each logical pair of sensor cells includes a firstsensor cell in the first sub-array, and a second sensor cell in thesecond sub-array. The first and second sensor cells can be located inthe same row of the array, or in different rows of the array.

In accordance with another embodiment of the present invention, animproved radiation sensor includes a first array of sensor cells and asecond array of sensor cells, wherein each of the sensor cells in thefirst array and each of the sensor cells in the second array includes aradiation sensitive capacitance structure and a read transistor coupledto a corresponding floating gate. Each of the sensor cells in the firstarray is logically paired with a corresponding one of the sensor cellsin the second array, wherein the logically paired sensor cells arespatially separated by a distance exceeding a correlation length ofradiation being sensed. A multiplexer circuit selectively couples readtransistors of logically paired sensor cells of the first and secondarrays in differential pairs to compare the threshold voltages of theread transistors. In one embodiment, the first array is identical to thesecond array.

In one embodiment, each of the differential pairs is coupled to acorresponding constant current source and a corresponding senseamplifier in order to compare the threshold voltages of the readtransistors.

In accordance with another embodiment, the radiation sensitivecapacitance structure of each sensor cell is designed to have arelatively small capacitance. Various structures can be used to achievethis small capacitance. In one embodiment, the floating gate of thesensor cell includes a plurality of polysilicon fingers in the radiationsensitive capacitance region. In another embodiment, each sensor cellincludes an air gap located adjacent to the radiation sensitivecapacitance structure. The air gap can be formed from either the frontside or the back side of the radiation sensor. In one embodiment, theair gap is located at the back side of the radiation sensor structure,and the air gap is sealed by a passivation layer or a bonded wafer. Inanother embodiment, the air gap is located in a multi-layer dielectricstructure located over the floating gate of the sensor cell, at thefront side of the radiation sensor structure.

In accordance with another embodiment, the radiation sensor includes amulti-layer interconnect structure that includes a plurality of metallayers and a plurality of dielectric layers. Radiation filters areformed over the radiation sensitive capacitance structures of the sensorcells, wherein the radiation filters are formed using the multi-layerinterconnect structure. In one embodiment, the radiation filters includetraces from one or more of the metal layers of the multi-layerinterconnect structure, and may comprise aluminum, copper and/ortungsten. In another embodiment, the radiation filters include one ormore of the dielectric layers of the multi-layer interconnect structure.These radiation filters advantageously allow the radiation sensor to beable to distinguish between radiation having different energies.

The present invention will be more fully understood in view of thefollowing description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a radiation sensor in accordance with oneembodiment of the present invention.

FIG. 2 is a circuit diagram illustrating C-Sensor cells and C-Referencecells, along with corresponding portions of a multiplexer circuit andsense amplifiers, in accordance with one embodiment of the presentinvention.

FIG. 3 is a top view of a C-sensor cell in accordance with oneembodiment of the present invention.

FIGS. 4A, 4B and 4C are cross sectional views of the C-sensor cell ofFIG. 3, along section lines A-A, B-B and C-C of FIG. 3, in accordancewith one embodiment of the present invention.

FIG. 5A is a top view of a C-sensor cell in accordance with an alternateembodiment of the present invention.

FIG. 5B is a cross-sectional view of the C-sensor cell of FIG. 5A, alongsection line B-B of FIG. 5A, in accordance with one embodiment of thepresent invention.

FIG. 6 is a cross-sectional view of a C-sensor cell in accordance withanother embodiment of the present invention.

FIG. 7A is a top view of a C-sensor cell in accordance with an alternateembodiment of the present invention.

FIG. 7B is a cross-sectional view of the C-sensor cell of FIG. 7A, alongsection line B-B of FIG. 7A, in accordance with one embodiment of thepresent invention.

FIG. 8 is a cross-sectional view of a C-sensor cell in accordance withanother embodiment of the present invention.

FIG. 9 is a cross-sectional view of a C-sensor cell in accordance withanother embodiment of the present invention.

FIG. 10A is a top view of a C-sensor cell in accordance with anotherembodiment of the present invention.

FIG. 10B is a cross-sectional view of the C-sensor cell of FIG. 10A,along section line B-B of FIG. 10A, in accordance with one embodiment ofthe present invention.

FIG. 11 is a block diagram of a radiation sensor that includes an arrayof unshielded C-sensor cells and an array of shielded C-sensor cells, inaccordance with another embodiment of the present invention.

FIG. 12A is a cross sectional view of a radiation sensing region of anunshielded C-sensor cell, which includes a dielectric structure having athickness of T1 over a floating gate, in accordance with one embodimentof the present invention.

FIG. 12B is a cross-sectional view of a radiation sensing region of ashielded C-sensor cell in accordance with one embodiment of the presentinvention.

FIG. 12C is a cross sectional view of a radiation sensing region of anunshielded C-sensor cell, which includes a dielectric structure having athickness of T2 over a floating gate, in accordance with one embodimentof the present invention.

FIG. 12D is a cross sectional view of a radiation sensing region of anunshielded C-sensor cell, which includes a dielectric structure having athickness of T3 over a floating gate, in accordance with one embodimentof the present invention.

DETAILED DESCRIPTION

In general, the present invention includes an ultra-sensitive radiationsensor and methods of operating the same. The radiation sensor does notneed a voltage supply in the registration (sensing) mode. In oneembodiment, the radiation sensor consists of an array of single polyfloating gate (FG) sensor cells with integrated-in-siliconlow-capacitance ionization chambers. To achieve high resolution ofradiation measurements (Gamma/X-ray radiation and energetic ions), thearray of sensor cells is divided into two parts. Each of the FG sensorcells has a corresponding FG sensor cell programmed to a similarthreshold voltage, thereby forming logical pairs of programmed FG sensorcells. Any difference in gate charges (manifested as changes inthreshold voltages) in the logical pairs of programmed FG sensor cellsis registered using a differential read operation. The number of logicalpairs of programmed FG sensor cells exhibiting a difference in gatecharge (i.e., the number of influenced pairs of FG sensor cells) is anindication of the absorbed radiation dose for low doses. The averaged FGcharge decrease of all the FG sensor cells is an indication of theabsorbed radiation dose at high doses. To account for influences notconnected with radiation in the low-dose measurement (temperature,natural retention loss of charge from FG), the threshold voltage (Vt)change of each FG sensor cell in the array is compared with the averagethreshold voltage change for all FG sensor cells in the array. The FGsensor cells with outlying threshold voltage changes are eliminated fromconsideration in the low-dose calculation. The present invention willnow be described in more detail.

FIG. 1 is a block diagram of a radiation sensor 100 in accordance withone embodiment of the present invention. Radiation sensor 100 includessensor cell array 101 that includes a plurality of single-poly floatinggate CMOS sensor (C-sensor) cells arranged in rows and columns. Ingeneral, the threshold voltages of the C-sensor cells will change inresponse to exposure to radiation (i.e., the C-sensor cells areradiation sensitive). As described in more detail below, the sensor cellarray 101 is logically divided into two sub-arrays 101A-101B forperforming low dose radiation measurements. Exemplary C-sensor cells 201and 202 are located in sub-arrays 101A and 101B, respectively. Decoder110 controls access to the C-sensor cells of sensor cell array 101 inthe manner described below, including program, erase and readoperations.

Radiation sensor 100 also includes reference cells 102, includingexemplary CMOS reference cells 211 and 212. In general, the thresholdvoltages of reference cells 102 will not change in response to exposureto radiation (i.e., the reference cells are radiation insensitive). Inone embodiment, reference cells 211 and 212 are non-programmed cells ofsame type as C-Sensor cells 201 and 202. In another embodiment,reference cells 211 and 212 are C-Sensor cells with floating gatesFG_(F1) and FG_(F2) shorted to control gates CG_(F1) and CG_(F2),respectively (as illustrated by dashed lines in FIG. 2).

In other embodiments, reference cells 211 and 212 are implemented bysingle-poly floating gate CMOS flash (C-Flash) transistors. In yet otherembodiments, the reference cells 102 can be implemented by resistors orMOS transistors. In the illustrated example, reference cells 211 and 212are aligned in the same ‘columns’ as C-sensor cells 201 and 202,respectively. Decoder 110 controls access to the reference cells 102 inthe manner described below.

Although reference cells 102 are shown as a single row of cells in theillustrated embodiment, it is understood that reference cells 102 can beimplemented as an array of cells in an alternate embodiment (whereineach of the C-sensor cells in array 101 has a corresponding referencecell in the array of reference cells). This array of reference cells canbe physically separated from sensor cell array 101. Alternately, thisarray of reference cells can be interspersed within the sensor cellarray 101. For example, reference cells 211 and 212 can be locatedimmediately adjacent to corresponding C-sensor cells 201 and 202,respectively, within the sensor cell array 101.

Decoder 110 also controls multiplexer circuit 120 to selectively couplethe C-sensor cell array 101 and reference cells 102 to sense amplifiers130 during read operations in a manner described in more detail below.Sense amplifiers 130 are coupled to provide analog read signals toanalog-to-digital converters (ADCs) 140.

FIG. 2 is a circuit diagram illustrating C-Sensor cells 201-202 andreference cells 211-212, along with corresponding portions ofmultiplexer circuit 120 and sense amplifiers 130, in accordance with oneembodiment of the present invention. C-sensor cells 201 and 202 aresingle-poly floating gate sensor devices, which include floating gatesFG_(S1) and FG_(S2), respectively, tunnel gates TG_(S1) and TG_(S2),respectively, control gates CG_(S1) and CG_(S2), respectively, and readtransistors RT_(S1) and RT_(S2), respectively (which include sourceregions S_(S1) and S_(S2), respectively, and drain regions D_(S1) andD_(S2), respectively). Various low-capacitance constructions forC-sensor cells 201 and 202 are described in more detail below. Ingeneral, the floating gates FG_(S1) and FG_(S2) are initially programmed(charged), such that the C-sensor cells 201 and 202 exhibit an initialthreshold voltage (Vt). If exposed to radiation, the charges stored bythe floating gates FG_(S1) and FG_(S2) are reduced, thereby reducing thethreshold voltages of the C-sensor cells 201 and 202.

Reference cells 211 and 212 are also single-poly floating gate devices,identical to the C-sensor cells 201 and 202, which include floatinggates FG_(F1) and FG_(F2), respectively, tunnel gates TG_(S1) andTG_(S2), respectively, control gates CG_(F1) and CG_(F2), respectively,and read transistors RT_(F1) and RT_(F2), respectively (which includesource regions S_(F1) and S_(F2), respectively, and drain regions D_(F1)and D_(F2), respectively). In low dose measurements (described in moredetail below), when a number of influenced pairs of C-sensor cells arecalculated, identical C-sensor cells are programmed to the samethreshold voltage to ensure identical dependence on temperature andaccount for natural threshold voltage (Vt) decrease. In high dosemeasurements (described in more detail below), the use of identicalC-sensor cells and reference cells also provide an advantage of the sametemperature response. As described above, C-sensor cells and referencecells can have the same design and layout, but the reference cells arenot programmed. Alternately, the C-sensor cells and reference cells canhave the same design and layout, but the control gates are connected tothe floating gates in the reference cells.

C-sensor cells 201-202 and reference cells 211-212 are coupled tomultiplexer transistors M₀-M₉ of multiplexer circuit 120. Morespecifically, the source S_(S1) and drain D_(S1) of C-sensor cell 201are coupled to multiplexer transistors M₀ and M₁, respectively; thesource S_(F1) and drain D_(F1) of reference cell 211 are coupled tomultiplexer transistors M₂ and M₃, respectively; the source S_(S2) anddrain D_(S2) of C-sensor cell 202 are coupled to multiplexer transistorsM₄ and M₅, respectively, and also to multiplexer transistors M₆ and M₇,respectively; and the source S_(F2) and drain D_(F2) of reference cell212 are coupled to multiplexer transistors M₈ and M₉, respectively.

Multiplexer transistors M₀-M₉ couple C-sensor cells 201-202 andreference cells 211-212 to sense amplifier circuits 221 and 222, asillustrated. More specifically, sense amplifier circuit 221 includes aconstant current source I_(C1), which is coupled to multiplexertransistors M₀, M₂ and M₄, a sense amplifier SA₁, which has one inputterminal coupled to multiplexer transistor M₁ and another input terminalcoupled to multiplexer transistors M₃ and M₅, a first drain resistor 231coupled between multiplexer transistor M₁ and the VDD voltage supplyterminal, and a second drain resistor 232 coupled between multiplexertransistors M₃ and M₅ and the VDD voltage supply terminal. In theillustrated embodiment, constant current source I_(C1) is implemented bya resistor 241 coupled to ground. The output of sense amplifier SA₁ isprovided to an analog-to-digital converter (ADC1).

Similarly, sense amplifier circuit 222 includes constant current sourceI_(C2), which is coupled to multiplexer transistors M₆ and M₈, senseamplifier SA₂, which has one input terminal coupled to multiplexertransistor M₇ and another input terminal coupled to multiplexertransistor M₈, a first drain resistor 233 coupled between multiplexertransistor M₇ and the VDD voltage supply terminal, and a second drainresistor 234 coupled between multiplexer transistor M₉ and the VDDvoltage supply terminal. In the illustrated embodiment, constant currentsource I_(C2) is implemented by a resistor 242 coupled to ground. Theoutput of sense amplifier SA₂ is provided to an analog-to-digitalconverter (ADC2).

The logical pair of C-sensor cells 201 and 202 are selected such thatthe physical distance (D1) between these cells within array 101 is largeenough that an energetic particle or photon that has an effect on one ofthese cells does not have an effect on the other one of these cells.That is, an energetic particle or photon that impinges on the C-sensorcell 201 (and changes the threshold voltage of this C-sensor cell 201)does not have any effect on the threshold voltage of the correspondingC-sensor cell 202 of the logical pair. Stated another way, the distanceD1 is greater than a correlation length of the energetic particle/photonof the radiation. Each of the C-sensor cells of sensor array 101 islogically paired with a corresponding C-sensor cell of array 101,wherein each logical pair of C-sensor cells is separated by a distanceof at least D1.

In an alternate embodiment, the physical distance D1 between logicalpairs of C-sensor cells is varied (using the multiplexer circuit 120) todefine the correlation length for different impinging particles. Forexample, for a first type of impinging particle, the distance D1 betweenthe C-sensor cells of a logical pair can be incrementally increased oversuccessive exposures, until an impinging particle only influences one ofthe C-sensor cells of the logical pair. The distance D1 under theseconditions defines the correlation length of the first type of impingingparticle.

In a particular embodiment, one of the C-sensor cells of a logical pairis located in sub-array 101A, and the other one of the C-sensor cells ofthe logical pair is located in sub-array 101B.

In one embodiment, the area of array 101 is large enough, and the areaof each C-sensor cell is small enough, to obtain statisticallysignificant results for low dose exposure. In one embodiment, the areaof array 101 is at least 10 mm² and the array 100 includes about 1 Mbitor fewer floating gate cells.

Although the logical pair of C-sensor cells 201 and 202 of the presentexample is shown in the same row of sensor cell array 101, it isunderstood that in other embodiments, paired C-sensor cells can belocated in different rows of sensor array 101. In one such anembodiment, sub-arrays 101A and 101B have independent decoder circuits,enabling different rows of sub-arrays 101A and 101B to be simultaneouslyaccessed.

The operation of radiation sensor 100, including paired C-sensor cells201-202 and corresponding reference cells 211-212, will now bedescribed. Sensor 100 is advantageously able to detect both low doseradiation and high dose radiation in the manner described below.

Initially, the C-sensor cells of C-sensor cell array 101 and referencecells 102 are programmed to a predetermined initial threshold voltageVT_(INIT). For example, the initial programmed threshold voltageVT_(INIT) may have a value in the range of about 2-4 Volts. In aparticular example, the floating gate of each C-sensor cell of array 101has an area of about 200 μm² over a shallow trench isolation (STI) layerhaving a thickness of about 3500 Angstroms, wherein the floating gatehas a capacitance of about 20 femto-farads (fF), and is charged to aninitial threshold voltage (V_(T)) of about 4V. Impingement of a singlealpha particle or gamma photon in the radiation sensitive capacitanceregion of the cell results in a corresponding threshold voltage changeon the order of approximately 1-2 mV.

Prior to exposure to radiation, the initial threshold voltage of each ofthe C-sensor cells in C-sensor cell array 101 is measured and recorded.This operation is performed by comparing the threshold voltage of eachC-sensor cell with the threshold voltage of the corresponding referencecell. For example, the initial threshold voltages of C-sensor cells 201and 202 are determined by performing comparisons with correspondingreference cells 211 and 212. In order to properly connect C-sensor cells201-202 and reference cells 211-212, read select signals RS_1 and RS_3are activated, thereby turning on multiplexer transistors M₀-M₃ andM₆-M₉. Under these conditions, C-sensor cell 201 and reference cell 211are commonly coupled to sense amplifier circuit 221, and C-sensor cell202 and reference cell 212 are commonly coupled to sense amplifiercircuit 222.

In the embodiment of FIG. 2, threshold voltage comparisons can beperformed by applying a fixed reference voltage to the control gatesCG_(F2) and CG_(F2) of reference cells 211 and 212, and changing thevoltages applied to the control gates CG_(S1) and CG_(S2) of C-sensorcells 201 and 202 until the moment when equal currents flow through thechannels of the C-sensor cells 201-202 and the corresponding referencecells 211-212. In one embodiment, closed loops are used to apply thevoltages to the control gates CG_(S1) and CG_(S2). More specifically,the outputs of sense amplifiers SA₁ and SA₂ are coupled to the controlgates CG_(S1) and CG_(S2), respectively, thereby creating feedback loopsthat automatically adjust the voltages applied to the control gatesCG_(S1) and CG_(S2), until equal currents flow through C-sensor cells201 and 202 and the corresponding reference cells 211 and 212. Underthese conditions, the final voltage provided by the sense amplifier SA₁is representative of the initial threshold voltage difference (ΔVT1_(INIT)) between C-sensor cell 201 and reference cell 211. Similarly,the final voltage provided by sense amplifier SA₂ is representative ofthe initial threshold voltage difference (ΔVT2 _(INIT)) between C-sensorcell 202 and reference cell 212.

The final voltages provided by sense amplifiers SA₁ and SA₂ are providedto ADC1 and ADC2, respectively, which convert these final voltages todigital threshold voltage values. This process is repeated for each rowof C-sensor cell array 101.

In an alternate embodiment, open loops are used to apply the voltages tothe control gates CG_(S1) and CG_(S2). In this embodiment, a firstdigital to analog converter (DAC) control circuit (not shown) is used toapply a first varying voltage to the control gate CG_(S1), and a secondDAC control circuit (not shown) is used to apply a second varyingvoltage to the control gate CG_(S2). The first and second DAC controlcircuits are also coupled to the outputs of sense amplifiers SA₁ andSA₂, respectively, thereby enabling the first DAC control circuit todetect when equal currents flow through the C-sensor cell 201 and thereference cell 211, and enabling the second DAC control circuit todetect when equal currents flow through the C-sensor cell 202 and thereference cell 212 (e.g., equal currents are indicated when the outputsof the sense amplifiers SA₁ and SA₂ change states). When equal currentsare detected in the C-sensor cell 201 and the reference cell 211, thecorresponding voltage provided by the first DAC control circuit to thecontrol gate CG_(S1) of C-sensor cell 201 is representative of theinitial threshold voltage difference (ΔVT1 _(INIT)) between C-sensorcell 201 and reference cell 211. Similarly, when equal currents aredetected in the C-sensor cell 202 and the reference cell 212, thecorresponding voltage provided by the second DAC control circuit to thecontrol gate CG_(S2) of C-sensor cell 202 is representative of theinitial threshold voltage difference (ΔVT2 _(INIT)) between C-sensorcell 202 and reference cell 212.

In various embodiments, the voltages provided by the first and secondDAC control circuits are varied by different selected algorithms, suchas successive approximation or single slope.

The initial threshold voltages of the C-sensor cells of sensor array 101will typically conform to a sharp Gaussian distribution, wherein mostC-sensor cells exhibit an average initial threshold voltage ofVT_(AVG_INIT).

Also prior to exposure to radiation, the initial threshold voltagedifferences between the logical pairs of C-sensor cells in sensor cellarray 101 are measured and recorded. For example, the initial thresholdvoltage difference between C-sensor cells 201 and 202 is determined asfollows. Read select signals RS_1 and RS_2 are activated, therebyturning on multiplexer transistors M₀-M₁ and M₄-M₅. Under theseconditions, C-sensor cell 201 and C-sensor cell 202 are coupled to senseamplifier circuit 221. The control gates CG_(S1) and CG_(S2) of C-sensorcells 201 and 202 are biased such that equal currents flow through theseC-sensor cells 201 and 202 (using either the closed loop method or theopen loop method described above). The difference between the voltagesapplied to the control gates CG_(S1) and CG_(S2) under this equalcurrent condition is measured, and is designated as the initialthreshold voltage difference (ΔVT12 _(INIT)) between C-sensor cell 201and C-sensor cell 202.

Note that it is desirable to decrease the total capacitance of theC-Sensor cells 201-202 to facilitate read (comparison) operations (i.e.,to ensure that small changes in threshold voltages due to impingement ofa single particle/photon can be detected). However, scaling down thesize of read transistors RT_(S1)-RT_(S2) is limited by the design rulesof corresponding CMOS technology. During a read (Vt comparison)operation, voltage applied to the control gate terminal is dividedbetween control gate to floating gate capacitance and the capacitance ofthe read transistor to ground. This voltage division limits the possibleinitial programming level (charge at the floating gate) of the C-Sensorcell, while the sensitivity of floating gate radiation sensors is knownto depend on the programming level. To eliminate the major part of readtransistors capacitances (Gate to Source) during a read (comparison)operation, equal constant currents are forced through read transistorsof C-sensor and reference cells. This leads to a constant potentialdifference between each of the floating gates and the channels of thecorresponding read transistors, thus effectively compensating the gatecapacitance of these transistors. The remaining capacitance to ground isgate to drain capacitance which is much smaller than the gate to sourcecapacitance. Technically, this is realized by the sense amplifiercircuit 221 of FIG. 2, wherein read transistors RT_(S1) and RT_(S2) arejoined into one differential pair biased by one tail current (providedby constant current source I_(C1)) to form the input stage to ADC1. Inequilibrium conditions, the currents are equal in each of the readtransistors RT_(S1) and RT_(S2) of the “logic pair”, while thedifference of voltages at the control gates CG_(S1) and CG_(S2) isrecorded by sense amplifier SA₁ and ADC1.

After the above-described threshold voltage measurements are taken andrecorded, radiation sensor 100 is exposed to radiation. Note that duringthe exposure, there are no external voltages applied to the C-sensorcells of sensor array 101 (or to the reference cells 102). That is, theC-sensor cells of sensor array 101 (and the reference cells 102) areused as passive sensitive elements when the sensor 100 is subjected toradiation.

After exposure, the threshold voltage differences between the logicalpairs of C-sensor cells are measured and recorded again, in the mannerdescribed above. For example, C-sensor cell 201 and C-sensor cell 202are biased in the manner described above and are coupled to senseamplifier circuit 221. In response, sense amplifier SA₁ provides anoutput to ADC1, which is representative of the post-exposure thresholdvoltage difference (ΔVT12 _(EXPOSED)) between C-sensor cell 201 andC-sensor cell 202.

The pre-exposure threshold voltage differences between the logical pairsof C-sensor cells are then compared with the post-exposure thresholdvoltage differences between the logical pairs of C-sensor cells. Forexample, the post-exposure threshold voltage difference between thelogical pair of C-sensor cells 201-202 (ΔVT12 _(EXPOSED)) is subtractedfrom the pre-exposure threshold voltage difference between the logicalpair of C-sensor cells 201-202 (ΔVT12 _(INIT)), thereby providing achange in the threshold voltage difference between this logical pair ofC-sensor cells 201-202 (ΔVT12 _(INIT)−ΔVT12 _(EXPOSED)). Note that for alow dosage exposure (less than about 1 mrad), it is likely that neitherof the logical pair of C-sensor cells 201-202 receives an alphaparticle/gamma photon. In this case, the change in the threshold voltagedifference (ΔVT12 _(INIT)−ΔVT12 _(EXPOSED)) should be 0 (because thethreshold voltages of C-sensor cells are not changed by the exposure).However, if one of the C-sensor cells 201-202 receives a single alphaparticle/gamma photon (e.g., from radon decomposition), and the otherone of the C-sensor cells does not, then the change in the thresholdvoltage difference (ΔVT12 _(INIT)−ΔVT12 _(EXPOSED)) should be a small,known value (e.g., 1 mV) based on the known design characteristics ofthe C-sensor cells.

All logical pairs of C-sensor cells that exhibit a change in thethreshold voltage difference corresponding with the known value (e.g., 1mV), are categorized as potentially influenced C-sensor cell pairs for alow dosage measurement. However, to confirm whether the potentiallyinfluenced C-sensor cell pairs are actually influenced, other factorsthat could lead to a change in the threshold voltage difference must beconsidered. More specifically, changes in threshold voltages due tonatural retention loss and changes in temperature must be considered. Toaccomplish this, the post-exposure threshold voltage of each of theC-sensor cells is measured and recorded, by comparing the thresholdvoltage of each C-sensor cell with the threshold voltage of thecorresponding reference cell, in the manner described above. Forexample, the post-exposure threshold voltage difference (ΔVT1_(EXPOSED)) between C-sensor cell 201 and reference cell 211, and thepost-exposure threshold voltage difference (ΔVT2 _(EXPOSED)) betweenC-sensor cell 202 and reference cell 212 are determined in the mannerdescribed above.

The post-exposure threshold voltages of the C-sensor cells of sensorarray 101 will typically conform to a sharp Gaussian distribution,wherein most C-sensor cells exhibit an average post-exposure thresholdvoltage of VT_(AVG_EXPOSED). The difference (ΔVT_(AVG)) or ‘shift’between the average initial threshold voltage VT_(AVG_INIT) and theaverage post-exposure threshold voltage of VT_(AVG_EXPOSED) isdetermined. This threshold voltage shift (ΔVT_(AVG)) is used todetermine whether the potentially influenced C-sensor cell pairs shouldbe counted as actually influenced C-sensor cell pairs. For example,assume that the threshold voltage shift (ΔVT_(AVG)) is 5 mV, whereinthis average threshold voltage shift is due to an external factor, suchas temperature. In this case, an actually influenced C-sensor cell pairis a C-sensor cell pair having a threshold voltage shift of 5 mV and athreshold voltage difference of 1 mV.

For example, assume that the initial (pre-exposure) threshold voltagesof C-sensor cells 201 and 202 are both measured at 0 Volts with respectto reference cells 211 and 212 (i.e., ΔVT1 _(INIT)=0 Volts; ΔVT2_(INIT)=0 Volts) and that the initial threshold voltage differencebetween C-sensor cells 201 and 202 is measured at 0 Volts (i.e., ΔVT12_(INIT)=0 Volts). Further assume that the post-exposure thresholdvoltage difference between C-sensor cells 201 and 202 is measured at 1mV (ΔVT12 _(EXPOSED)=1 mV), such that the logical pair of C-sensor cells201 and 202 is identified as a potentially influenced C-sensor cell pair(i.e., ΔVT12 _(INIT)−ΔVT12 _(EXPOSED)=1 mV). Further assume that thethreshold voltage shift ΔVT_(AVG) is determined to be 5 mV. In thiscase, the post-exposure threshold voltage difference (ΔVT1 _(EXPOSED))between C-sensor cell 201 and reference cell 211, and the post-exposurethreshold voltage difference (ΔVT2 _(EXPOSED)) between C-sensor cell 202and reference cell 212 are analyzed to determine whether the potentiallyinfluenced C-sensor cell pair 201-202 are identified as an actuallyinfluenced C-sensor cell pair.

For example, if the post-exposure threshold voltage difference ofC-sensor cell 201 (ΔVT1 _(EXPOSED)) is equal to 5 mV, and thepost-exposure threshold voltage difference of C-sensor cell 202 (ΔVT2_(EXPOSED)) is equal to 6 mV, then the C-sensor cell pair 201-202corresponds with an actually influenced C-sensor cell pair, becausethese post-exposure threshold voltage differences correspond with thecalculated threshold voltage shift ΔVT_(AVG) of 5 mV and the expectedpost-exposure threshold voltage difference of 1 mV.

However, if the post-exposure threshold voltage difference of C-sensorcell 201 (ΔVT1 _(EXPOSED)) is equal to 10 mV, and the post-exposurethreshold voltage difference of C-sensor cell 202 (ΔVT2 _(EXPOSED)) isequal to 9 mV, then the C-sensor cell pair 201-202 does not correspondwith an actually influenced C-sensor cell pair, because thesepost-exposure threshold voltage differences do not correspond with thecalculated threshold voltage shift ΔVT_(AVG) of 5 mV.

If the post-exposure threshold voltage difference of C-sensor cell 201(ΔVT1 _(EXPOSED)) is equal to 7 mV, and the post-exposure thresholdvoltage difference of C-sensor cell 202 (ΔVT2 _(EXPOSED)) is equal to 5mV, then the C-sensor cell pair 201-202 does not correspond with anactually influenced C-sensor cell pair, because these post-exposurethreshold voltage differences do not correspond with the expectedpost-exposure threshold voltage difference of 1 mV.

In accordance with one embodiment, the number of actually influencedC-sensor cell pairs is used to determine the dosage of the low doseexposure. That is, each actually influenced C-sensor cell paircorresponds with a single received alpha particle/gamma photon. Thelocations of the actually influenced C-sensor cell pairs within array101 (as indicated by the row/column addresses of the actually influencedC-sensor cell pairs) can also be used to define the spatial distributionof the received low dose radiation. The area affected by a single alphaparticle/gamma photon can also be estimated in response to the number ofadjacent actually influenced C-sensor cell pairs.

Note that the 1/f noise performance is critical for the low dosageoperation of radiation sensor 100. Noises below the official Spicevalues for the employed technology are achieved by keeping the readouttransistors in strong accumulation or inversion regimes and chopping tobring them into the read-out mode. After exposure to low doses ofradiation, large charges will remain stored in the floating gates of theC-sensor cells of array 101. Under these conditions, the SiO₂ interfacetraps of the C-sensor cells are continuously filled (or empty), andthus, 1/f noise is suppressed. That is, the readout transistors of theC-sensor cells are placed in a strong accumulation regime (or a stronginversion regime). Advantageously, the method described above does notrequire cycling pulses to shift the C-sensor cells intoaccumulation/inversion. Note that for large radiation doses (describedin more detail below), the readout signals are large and 1/f noise isnot critical.

In accordance with one embodiment, the above-described low dosemeasurement method is used to determine the dosage of the receivedradiation as long as the number of actually influenced C-sensor cellpairs is less than a predetermined percentage (e.g., 50%) of the totalnumber of C-sensor cell pairs in sensor 100. Other percentages can beused in other embodiments.

If the number of actually influenced C-sensor cell pairs is greater thanhalf the total number of C-sensor cell pairs, then the average thresholdvoltage shift (ΔVT_(AVG)) can be used to calculate the high doseexposure. In one embodiment, the pre-exposure threshold voltagedifference (ΔVT1 _(INIT)) between C-sensor cell 201 and C-reference cell211 is compared with the post-exposure threshold voltage difference(ΔVT1 _(EXPOSED)) between C-sensor cell 201 and C-reference cell 211 todetermine a threshold voltage shift (i.e., ΔVT1 _(INIT)−ΔVT1_(EXPOSED)=ΔVT1 _(SHIFT)). The threshold voltage shift for all C-Sensorcells and their corresponding C-reference cells are determined in thesame manner, and the average threshold voltage shift ΔVT_(AVG) iscalculated (i.e., average ΔVT_(AVG)=Σ ΔV_(T_SHIFT) of all C-sensorcells/Number of C-sensor cells). The absorbed radiation is thendetermined in response to the average threshold voltage shift ΔVT_(AVG).Note that under high dose exposure, the threshold voltage shift due tonatural retention loss and temperature effects will generally representan insignificant portion of the average threshold voltage shiftΔVT_(AVG).

Reducing the specific (normalized to area) capacitance of the C-sensorcell will increase the change of voltage on the control gate for a givenabsorbed charge. It is therefore desirable to reduce the capacitance ofthe pixel sensing volume of the C-sensor cells of array 101 to asufficiently low value, such that impingement of a single particle willresult in a measurable threshold voltage change (e.g., about 1 mV).

Accordingly, the present invention also includes several embodiments ofC-sensor cells having a reduced floating gate capacitance in the pixelsensing region. These embodiments are described in more detail below.

FIG. 3 is a top view of C-sensor cell 300 in accordance with oneembodiment of the present invention. FIGS. 4A, 4B and 4C are crosssectional views of the C-sensor cell 300 along section lines A-A, B-Band C-C of FIG. 3. In this embodiment, C-sensor cell 300 includessubstrate 350, deep n-well region 301, n-well regions 302-307, p-wellregions 310, 320 and 330, shallow trench isolation (STI) region 315, P+contact regions 311 and 321, N+ contact regions 312 and 322, thin gatedielectric regions 325 and 335, N-type drain region 331 (D_(S1)) andN-type source region 332 (S_(S1)), as illustrated. The P-well region 310forms the control gate CG_(S1), wherein connections to the control gateCG_(S1) are provided via P+ region 311 and N+ region 312. Similarly, thep-well region 320 forms the tunnel gate TG_(S1), wherein connections tothe tunnel gate TG_(S1) are provided via P+ region 321 and N+ region322. The floating gate FG_(S1) extends over P-well regions 310, 320 and330, wherein the floating gate FG_(S1) is separated from the P-wellregion 310 (i.e., control gate CG_(S1)) by a relatively thick STI region315 having a thickness of about 3500 Angstroms, the floating gateFG_(S1) is separated from the P-well region 320 (i.e., tunnel gateTG_(S1)) by a relatively thin gate dielectric region 325 having athickness of about 100 Angstroms, and the floating gate FG_(S1) isseparated from the P-well region 330 (i.e., channel region of the readtransistor RT_(S1)) by a relatively thin gate dielectric region 335having a thickness of about 100 Angstroms.

The portion of the floating gate FG_(S1) that extends over the p-wellregion 310 (i.e., control gate CG_(S1)) includes a plurality of parallelrectangular fingers F₁-F₅, which reduces the overlap area of thefloating gate FG_(S1) over the control gate CG_(S1), therebyadvantageously reducing the capacitance between the floating gateFG_(S1) and the control gate CG_(S1) (when compared with a conventionalrectangular floating gate structure that extends entirely over thep-well region 310). This low capacitance in the pixel sensing regionadvantageously enables the threshold voltage of the read transistorRT_(S1) to be more sensitive to changes in the charge stored by thefloating gate FG_(S1). In one embodiment, the C-sensor cell 300 of FIGS.3 and 4A-4C enables the threshold voltage of the read transistor RT_(S1)to change by 1 mV in response to the absorption of a single alphaparticle/gamma photon in the STI region 315 (i.e., the pixel sensingvolume of the radiation sensitive capacitance region).

FIG. 5A is a top view of a C-sensor cell 500 in accordance with analternate embodiment of the present invention. FIG. 5B is across-sectional view of C-sensor cell 500 along section line B-B of FIG.5A. Because C-sensor cell 500 is similar to C-sensor cell 300, similarelements in FIGS. 3, 4A-4C and 5A-5B are labeled with similar referencenumbers. As illustrated by FIGS. 5A and 5B, the control gate CG_(S1) ofC-sensor 500 is implemented by a metal structure 510, which is formed ona dielectric layer 502 over the floating polysilicon gate FG_(S1).Although the portion of the floating gate FG_(S1) located under themetal control gate CG_(S1) has a solid rectangular shape in theillustrated embodiment, it is understood that this portion of thefloating gate FG_(S1) can be modified to include a plurality of parallelfingers (e.g., similar to fingers F1-F5) in other embodiments. Inanother embodiment, the metal control gate CG_(S1) can be implemented bya plurality of parallel fingers, or a mesh structure. These alternateembodiments may be used to reduce the capacitance between the controlgate CG_(S1) and the underlying floating gate FG_(S1).

To further reduce the capacitance associated with the control gateCG_(S1)/floating gate FG_(S1) structure, a series of etches areperformed to create a cavity (i.e., air gap) 505 under the control gateCG_(S1) and floating gate FG_(S1). Cavity 505 can be formed inaccordance with various processing techniques, including those describedin commonly owned, co-pending U.S. patent application Ser. No.16/246,550 to Sirkis et al., titled “Semiconductor Device Having a RadioFrequency Circuit and a Method For Manufacturing the SemiconductorDevice”, filed Jan. 14, 2019, which is hereby incorporated by referencein its entirety. In general, cavity 505 can be formed by etching fromthe front side or the back side of the wafer structure. For example, toetch cavity 505 from the back side of substrate 350, a mask (not shown)is formed over the back side surface of substrate, and etch is performedthrough an opening in this mask to form a hollow 501, wherein the hollow501 is a hole having a high aspect ratio. In one embodiment, this etchis a reactive ion etch that implements a Bosch process. The sidewalls ofthe hollow 501 are coated with a polymer 502 in the process of Boschetch. A series of etches are then performed through the hollow 501 (andpolymer 502) to remove portions of substrate 350 and STI layer 315 toexpose the lower surface of floating gate FG_(S1) and create cavity 505.In one embodiment, the series of etches includes a reactive ion etchthat changes the chemistry of the Bosch etch used to form hollow 501.Note that this series of etches does not remove the polymer 502, whichfunctions as an etch stop (or the polysilicon floating gate FG_(S1)).The interior of cavity 505 is then passivated, and the hollow 501 isthen sealed (e.g., by forming a passivation layer 507 over the back sidesurface of substrate 301). Note that a similar process can be used toform cavity 505 from the front side of the wafer structure. The airgap/ionization chamber formed by cavity 505 effectively reduces thecapacitance associated with the portion of the floating gate FG_(S1)located under the metal control gate CG_(S1). Note that this ionizationchamber forms a pixel sensing volume.

FIG. 6 is a cross-sectional view of a C-sensor cell 600 in accordancewith another embodiment of the present invention. Because C-sensor cell600 is similar to C-sensor cell 500, similar elements in FIGS. 5A-5B and6 are labeled with similar reference numbers. C-sensor cell 600 has asilicon-on-insulator (SOI) structure, wherein STI region 315 is formedon a buried oxide layer 602, which in turn, is located on a handle wafer601. In this embodiment, the tunnel gate structure TG_(S1) and readtransistor RT_(S1) are formed in silicon islands (such that the deepn-well 301 and n-well regions 302-307 of FIGS. 4A-4C are not required).Cavity 615 is formed through the handle wafer 601, and extends to theburied oxide layer 602 under the control gate CG_(S1), as illustrated.Cavity 615 is formed through hollow 611 and polymer (etch stop) lining612 in the manner described above in connection with FIG. 5B. Hollow 611is sealed by passivation layer 617, such that cavity 615 forms an airgap/ionization chamber that reduces the capacitance of the control gateCG_(S1)/floating gate FG_(S1) structure.

FIG. 7A is a top view of a C-sensor cell 700 in accordance with analternate embodiment of the present invention. FIG. 7B is across-sectional view of C-sensor cell 700 along section line B-B of FIG.7A. C-sensor cell 700 has a SOI structure, wherein STI region 703 isformed on a buried oxide layer 702, which in turn, is located on ahandle wafer 701. The read transistor RT_(S1), which is formed in firstsilicon island, includes p-type body region 711 and n-type source/drainregions 712-713. Floating gate FG_(S1) is formed in a second siliconisland, which includes p-type region 715. Floating gate FG_(S1) islaterally separated from the p-type body region 711 by a portion of STIregion 703. The edge 711C of p-type body region 711 adjacent to thefloating gate FG_(S1) forms a channel region of the read transistorRT_(S1). The amount of charge stored by floating gate FG_(S1) influencesthe channel region 711C (and thereby the read current) of readtransistor RT_(S1). A dielectric layer 704 is formed over the floatinggate FG_(S1), the read transistor RT_(S1) and the STI region 703, asillustrated. A polysilicon structure 705 is formed over the dielectriclayer 704 (and over the floating gate FG_(S1)), wherein the polysiliconstructure 705 forms the tunneling gate TG_(S1). A dielectric structure729 is formed over the polysilicon structure 705, and a metal structure730 is formed over the dielectric structure 729 (and over the floatinggate FG_(S1)), wherein the metal structure 730 forms the control gateCG_(S1). The floating gate FG_(S1) can be programmed/erased, by applyingvoltages to the tunneling gate TG_(S1) and control gate CG_(S1).

C-sensor cell 700 also includes a cavity 720, which extends through thehandle wafer 701 to the BOX layer 702 under the floating gate FG_(S1).Cavity 720 is formed in the same manner described above for cavity 615(i.e., forming hollow 721 and polymer (etch stop) coating 722, etchingcavity 720 through hollow 721, and sealing hollow 721 with passivationlayer 723). The resulting air gap advantageously reduces the capacitanceof the control gate CG_(S1)/floating gate FG_(S1) structure.

FIG. 8 is a cross-sectional view of a C-sensor cell 800 in accordancewith another embodiment of the present invention. Because C-sensor cell800 is similar to the C-sensor cell 300, similar elements in FIGS. 3,4A-4C and 8 are labeled with similar reference numbers. Thus, C-sensorcell 800 includes substrate 350, deep n-well 301, p-well regions 310,320 and 330, P-type regions 311 and 321, n-type regions 312, 322 and331-332, and gate dielectric regions 325 and 335. In C-sensor cell 800,the floating gate FG_(S1) is separated from the P-well region 310 bygate dielectric region 805, which has the same thickness as gatedielectric regions 325 and 335 (e.g., 100 Angstroms). STI region 815,which has a relatively large thickness in the range of about 3500Angstroms, is located over ionization chamber 801. In one embodiment,ionization chamber 801 is formed by performing a backside etch throughsubstrate 350 to STI region 815, and then bonding a dummy wafer 810 tothe backside of substrate 350. Electrons are excited from the walls ofionization chamber 801 by gamma photons and optionally produceionization of the air in chamber 801. The created charges reduce thecharge stored by floating gate FG_(S1). Dummy wafer 810 may be thinenough to allow the passage of alpha particles, or alpha particles maybe detected from the front side of C-sensor cell 800.

FIG. 9 is a cross-sectional view of a C-sensor cell 900 in accordancewith another embodiment of the present invention. Because C-sensor cell900 is similar to the C-sensor cell 500 of FIGS. 5A-5B, similar elementsin FIGS. 5A-5B and 9 are labeled with similar reference numbers. Notethat in C-sensor cell 900, the cavity 505 described above in connectionwith FIG. 5B is stopped on the STI region 315 (rather than extendingthrough STI region 315). Also in C-sensor cell 900, a cavity 901 isformed from the front side of the wafer structure, through thedielectric structure 502 (which may include several dielectric layers).In one embodiment, cavity 901 is formed at the stage of pad etch. Cavity901 exposes the portion of the floating gate FG_(S1) located over cavity505. A metal grill (or mesh) 910 is formed over cavity 901, therebyforming control gate CG_(S1). Cavities 505 and 901 advantageously reducethe capacitance of the control gate CG_(S1)/floating gate FG_(S1)structure. The capacitance lowering effect is increased because materialis removed from both sides of the floating gate FG_(S1).

FIG. 10A is a top view of a C-sensor cell 1000 in accordance withanother alternate embodiment of the present invention. FIG. 10B is across-sectional view of C-sensor cell 1000 along section line B-B ofFIG. 10A. Because C-sensor cell 1000 includes elements similar to thosefound in C-sensor cells 300, 600 and 900, similar elements in FIGS. 3,4A-4C, 6, 9 and 10A-10B are labeled with similar reference numbers. Asillustrated by FIG. 10A, C-sensor cell 1000 includes many of the sameelements described above in connection with FIG. 3. Note however, thatthe polysilicon floating gate FG_(S1) of C-sensor cell 1000 extends pastthe control gate CG_(S1) to an extension region 1001.

As illustrated by FIG. 10B, a thin gate dielectric region 1015 (ratherthan the STI region 315) is located between the floating gate FG_(S1)and the underlying control gate region. Note that the control gateregion CG_(S1), the tunnel gate region TG_(S1) and the read transistorRT_(S1) are formed in corresponding silicon islands, which are locatedon buried oxide layer 602, and surrounded by STI region 315.

Over the floating gate extension region 1001, a cavity 901 is formedthrough the dielectric structure 502 to expose a portion of the floatinggate extension region. A metal grid (or mesh) 1010 is located over thecavity 901 (in the same manner that metal grid 910 is located overcavity 901 in C-sensor cell 900). Metal grid 1010 is grounded. In thisembodiment, there is a separate isolated control gate CG_(S1).

Also, under the floating gate extension region 1001, an airgap/ionization chamber 615 is formed through handle wafer 601 to buriedoxide layer 602 (wherein cavity 615 is formed through hollow 611 andsidewall etch stop layer 612, and capped by passivation layer 617 in themanner described above in connection with FIG. 6). During exposure toradiation, charges generated by alpha particles/gamma photons mayapproach STI 315 within extension region 1001 via ionization chamber 615or floating gate FG_(S1) via cavity 901. The capacitance of theassociated pixel sensing volume is low because the removal of materialfrom both sides of the floating gate extension region 1001.

The low capacitances of the pixel sensing volumes of C-sensor cells 300,500, 600, 700, 800, 900 and 1000 advantageously enable the use of theseC-sensor cells in the sensor array 101 of radiation sensor 100 inaccordance with various embodiments of the present invention.

Absorption of the same doses of radiation having different energy mayhave different impact on solid state devices. This means that the samedose of radiation, but from particles with different energies, willresult in different threshold voltage (Vt) shifts in the C-sensor cellsof array 101. This may result in a mistake in estimating the absorbeddose of radiation. In accordance with one embodiment, this obstacle isovercome by using two (or more) different types of C-sensor cells,wherein at least one of these types of C-sensor cells includes aradiation filter (shield). Simultaneous exposure of two differentC-sensor cells (one with a filter and one without a filter) will lead todifferent responses (i.e., different changes in threshold voltages),because some portion of the radiation is absorbed in the filter. Becausethe absorption of different radiation energies is different in filtermaterials, the difference in response of shielded and non-shieldedsensors may be used to define both the energy and dose of absorbedradiation. In the prior art, shielding is implemented by using differentpackages or covering materials. In accordance with one embodiment of thepresent invention, C-sensor cells are shielded using materials which area part of the standard CMOS process, such as aluminum, copper, tungstenand dielectric material.

FIG. 11 is a block diagram of a radiation sensor 1100 that includes anarray 1101 of C-sensor cells, which includes the array 101 of C-sensorcells described above (which includes identical sub-arrays 101A and101B). In addition, the C-sensor cell array 1101 of FIG. 11 alsoincludes a second array 103 of C-sensor cells (which includes identicalsub-arrays 103A and 103B), and a third array 105 of C-sensor cells(which includes identical sub-arrays 105A and 105B). In general,C-sensor cell arrays 103 and 105 operate in the same manner describedabove in connection with C-sensor cell array 101. However, the C-sensorcells of arrays 103 and 105 include different radiation filteringstructures than the C-sensor cells or array 101, in the manner describedin more detail below.

FIG. 12A is a cross sectional view of the unshielded C-sensor cell 300of FIGS. 3 and 4A, which includes a dielectric structure 1205 having athickness of T1 over the floating gate FG_(S1). Dielectric structure1205 includes the various dielectric layers deposited during theformation of the multi-layer interconnect structure (note that the metallayers of this multi-layer interconnect structure are not included indielectric structure 1205). As described above, C-sensor cell 300 isincluded in C-sensor cell array 101.

FIG. 12B is a cross-sectional view of a shielded C-sensor cell 1201(along the same section line as FIG. 12A) in accordance with oneembodiment of the present invention. Similar elements in FIGS. 12A and12B are labeled with similar reference numbers. Notably, shieldedC-sensor cell 1201 includes a radiation filter (shield) 1210 over theradiation sensitive capacitance structure associated with the floatinggate FG_(S1), wherein the filter 1210 includes aluminum layers 1211-1213and tungsten layers 1221-1222, which are layered as illustrated. Inaccordance with one embodiment, aluminum layers 1211-1213 are formed atthe same time as metal layers M1-M3 of the multi-layer interconnectstructure, and tungsten layers 1221 and 1222 are formed at the same timeas the inter-metal contacts between metal layers M1-M2 and M2-M3. As aresult, no additional process steps are required to form filter 1210.Although filter 1210 includes three aluminum layers 1211-1213 and twotungsten layers 1221-1222, it is understood that other numbers of layerscan be used in other embodiments to adjust the radiation absorbingproperties of the filter 1210. It is also understood that in otherembodiments, filter 1210 can be implemented using other materialscommonly available in a CMOS process (e.g., copper).

FIGS. 12C and 12D are cross sectional views of C-sensor cells 1202 and1203 respectively (along the same section line as FIG. 12A) inaccordance with other embodiments of the present invention. Similarelements in FIGS. 12A, 12C and 12D are labeled with similar referencenumbers. Notably, C-sensor cell 1202 includes a dielectric structure1206 having a thickness of T2 over floating gate FG_(S1), wherein thethickness T2 is less than the thickness T1 of dielectric structure 1205.Similarly, C-sensor cell 1203 includes a dielectric structure 1207having a thickness of T3 over floating gate FG_(S1), wherein thethickness T3 is less than the thickness T2 of dielectric structure 1206.In accordance with one embodiment, dielectric structures 1206 and 1207can be fabricated by eliminating one or more dielectric layers of themulti-layer interconnect structure in the area over floating gateFG_(S1). As a result, no additional processing steps are required toimplement dielectric structures 1205-1207. Assuming that dielectricstructures 1205, 1206 and 1207 are constructed of the same material(s)(e.g., silicon oxide), C-sensor cells 201, 1202 and 1203 willadvantageously exhibit different sensitivity to radiation havingdifferent energies (effectively providing a shielding/filter function).

In accordance with various embodiments of the present invention, theC-sensor cells 1201, 1202 and 1203 (and the above-described variationsthereof) can be used to implement the C-sensor cells included in sensorarrays 103 and 105 within sensor 1100 (FIG. 11). For example, C-sensorarray 103 can be implemented using C-sensor cell 1201, and C-sensorarray 105 can be implemented using C-sensor cell 1202 (or 1203). Inanother example, C-sensor array 103 can be implemented using C-sensorcell 1201, and C-sensor array 105 can be implemented using a modifiedversion of C-sensor cell 1201, which includes different metal layersthan C-sensor cell 1201 in the manner described above. In anotherexample, C-sensor array 103 can be implemented using C-sensor cell 1202,and C-sensor array 105 can be implemented using C-sensor cell 1203.While the C-sensor cell array 1101 of FIG. 11 includes three C-sensorcell arrays 101, 103 and 105, it is understood that in otherembodiments, C-sensor cell array 1101 can be modified to include onlytwo C-sensor cells arrays (e.g., C-sensor cell arrays 101 and 103), ormore than three C-sensor cell arrays.

Providing C-sensor cell arrays 101, 103, 105 having different radiationfilters advantageously allows radiation sensor 1100 to effectivelymeasure radiation having different energies.

Although the invention has been described in connection with severalembodiments, it is understood that this invention is not limited to theembodiments disclosed, but is capable of various modifications, whichwould be apparent to a person skilled in the art. Thus, the invention islimited only by the following claims.

We claim:
 1. A method for radiation dosage measurement comprising:exposing a plurality of sensor cells to radiation, thereby providing acorresponding plurality of exposed sensor cells, wherein each of theplurality of sensor cells includes a single-polysilicon floating gate;performing a plurality of differential read operations, eachdifferential read operation accessing a corresponding logical pair ofthe exposed sensor cells, and each differential read operationidentifying a threshold voltage difference between the correspondinglogical pair of the exposed sensor cells; determining whether eachlogical pair of the exposed sensor cells is influenced by exposure tothe radiation in response to the corresponding identified thresholdvoltage difference; and determining a dosage of the radiation inresponse to the number of logical pairs of the exposed sensor cellsdetermined to be influenced by exposure to the radiation.
 2. The methodof claim 1, wherein each of the plurality of sensor cells includes aread transistor, wherein each of the plurality of differential readoperations includes coupling the read transistor of each exposed sensorcell of a corresponding logical pair of the exposed sensor cells to aconstant current source.
 3. The method of claim 1, wherein each of theplurality of sensor cells includes a read transistor, wherein each ofthe plurality of differential read operations includes coupling the readtransistor of each exposed sensor cell of a corresponding logical pairof the exposed sensor cells to a common sense amplifier.
 4. The methodof claim 1, further comprising: determining an average threshold voltageshift of the plurality of sensor cells that is unrelated to theradiation; and using the average threshold voltage shift to determinewhether each logical pair of the exposed sensor cells is influenced byexposure to the radiation.
 5. The method of claim 1, wherein determiningwhether each logical pair of the exposed sensor cells is influenced byexposure to the radiation includes: determining whether the identifiedthreshold voltage difference of the logical pair of the exposed sensorcells has a predetermined value.
 6. The method of claim 1, furthercomprising: initializing charges stored by the floating gates of each ofthe plurality of sensor cells prior to exposing the plurality of sensorcells to radiation, thereby providing a plurality of initialized sensorcells.
 7. The method of claim 6, wherein each of the plurality of sensorcells includes a read transistor, and initializing charges stored by thefloating gates of each of the plurality of sensor cells suppresses 1/fnoise of the read transistors by ensuring that traps associated with theread transistors are all filled or all empty.
 8. The method of claim 1,further comprising determining whether the number of logical pairs ofthe exposed sensor cells determined to be influenced by exposure to theradiation exceeds a threshold number.
 9. The method of claim 8, furthercomprising calculating the dosage of the radiation based on the numberof logical pairs of the exposed sensor cells determined to be influencedby exposure to the radiation if the number of logical pairs of theexposed sensor cells determined to be influenced by exposure to theradiation does not exceed the threshold number.
 10. The method of claim9, further comprising calculating the dosage of the radiation based on adifference between an average threshold voltage of the plurality ofsensor cells prior to exposure to the radiation and an average thresholdvoltage of the exposed sensor cells, if the number of logical pairs ofthe exposed sensor cells determined to be influenced by exposure to theradiation exceeds the threshold number.
 11. The method of claim 8,wherein the threshold number represents half of the plurality of sensorcells.
 12. The method of claim 1 where the radiation comprisesgamma/X-ray radiation and particles having energies in the range of 10keV to 100 MeV.
 13. The method of claim 1, further comprising logicallydividing the plurality of sensor cells into a first array and a secondarray, where each logical pair of the exposed sensor cells includes afirst sensor cell in the first array, and a second sensor cell in thesecond array.
 14. The method of claim 13, wherein the first and secondsensor cells are located in the same rows of the first and secondarrays.
 15. The method of claim 1, wherein the exposed sensor cells ofeach logical pair of the exposed sensor cells are spatially separated bya distance that exceeds a correlation length of an energeticparticle/photon of the radiation.
 16. The method of claim 1, furthercomprising: prior to exposing the plurality of sensor cells toradiation, determining an initial threshold voltage of each of theplurality of sensor cells.
 17. The method of claim 16, wherein theinitial threshold voltage of each of the plurality of sensor cells isdetermined by performing a differential read operation with a radiationinsensitive reference cell.
 18. The method of claim 17, wherein eachradiation insensitive reference cell is identical to each of theplurality of sensor cells, and wherein the method further comprisesinitially programming each of the plurality of sensor cells, and notprogramming each radiation insensitive reference cell.
 19. The method ofclaim 17, wherein each radiation insensitive reference cell has acontrol gate electrically connected to a floating gate.
 20. The methodof claim 17, wherein each radiation insensitive reference cell comprisesa CMOS flash cell.
 21. The method of claim 16, further comprising usingthe initial threshold voltage of each of the plurality of sensor cellsto determine an average initial threshold voltage of all of theplurality of sensor cells.
 22. The method of claim 21, furthercomprising determining the threshold voltage of each of the plurality ofexposed sensor cells, and using the threshold voltage of each of theplurality of exposed sensor cells to determine an average post-exposurethreshold voltage of all of the plurality of exposed sensor cells. 23.The method of claim 22, wherein the threshold voltage of each of theplurality of exposed sensor cells is determined by performing adifferential read operation with a radiation insensitive reference cell.24. The method of claim 22, further comprising determining an averagethreshold voltage shift by determining the difference between theaverage initial threshold voltage and the average post-exposurethreshold voltage.
 25. The method of claim 24, further comprising usingthe average threshold voltage shift to determine whether each logicalpair of the exposed sensor cells is influenced by exposure to theradiation.